Fluid separation is an expensive energy consuming process. Industrial separation of gas mixtures of O2/N2, CO2/CH4, H2/N2, and Olefin/Paraffin are particularly expensive and have been commonly undertaken industrially by distillation and pressure sweep processes. An alternative is to use membranes such as a carbon molecular sieve membranes (CMSMs).
CMSMs are highly porous materials distributed with small selective pores. These pores are in the order of 3-6 Å in diameter, making the pores similar in dimension to diffusing gas molecules. The small pore size restricts the rate at which gasses of different size and shape move across the membrane, thereby allowing the CMSMs to exhibit selective adsorption and molecular sieving capabilities for gas mixtures. Accordingly, CMSM can be used to effect size and shape separation between gas molecules of similar molecular dimensions.
CMSMs membranes are usually prepared by pyrolysis of dense polymeric precursors that already show intrinsic gas separation properties. CMSMs exhibit enhanced mass transport properties and separation selectivity of gas mixtures compared to their polymer precursors. Furthermore CMSMs have also been recognized as having higher thermal and chemical stability, making them suitable for use in corrosive, high pressure and high temperature environments.
Studies have reported that a CMSM with tailored microstructure (pore size, pore volume, etc) could be obtained by controlling the pyrolysis conditions and post-/pre-treatment conditions. Thus, attention has been focused on modification of carbon membranes to optimize the separation efficiency. According to reported studies, the current modification technologies performed on carbon membranes include thermo-stabilisation, oxidation, chemical vapor deposition, chemical treatment and physical stretching. One proposal is a first modification method on CMSM by altering pore openings by oxidation and sintering. It was found that the permeability of CMSM increased the oxidized membranes whereas lower permeability was observed for sintered membranes. Other studies have explored a changing the pore dimension of carbon membranes by calcination under mild activation to exhibit excellent selectivity for alkene/alkane separation. On the other hand, other studies have successfully controlled the pore sizes and increased the selectivity of carbon membranes by chemical vapor deposition of propylene.
One study also performed the oxidation on resulting carbon membranes. It was suggested that oxidation has broadened the pore size distribution which resulted in the increment of permeability accompanied by a slight decline in selectivity. In yet another study, oxidation increased the permeances without sacrificing the selectivity of carbon membranes. All above mentioned modifications were carried out on resulting carbonized membranes after pyrolysis, but not on polymer precursors.
One study involved thermal treatment of polymeric hollow fibers in atmospheric air at 400° C. for 30 minutes before pyrolysis. It was found that thermo-stabilisation process strengthened the structure of precursors in order to withstand the high temperatures during pyrolysis.
U.S. Pat. No. 4,919,860 proposed a chemical pretreatment using chemical reagent, where the capillary acrylonitrile membranes were pre-treated in aqueous hydrazine solution before carbonization. It was found that the hydrazine pretreatment improved the dimensional stability of the membrane and at the same time prevented tar formation and clogging of pores during the pyrolysis step.
Another study disclosed in Chen, J. C.; Harrison, I. R. Modification of polyacrylonitrile (PAN) carbon fiber precursor via post-spinning plasticization and stretching in dimethylformamide (DMF), Carbon 2002, 40, 25, modified polyacrylonitrile (PAN) carbon fiber precursors via physical stretching in dimethylformamide (DMF). The post-spinning plasticization and stretching process was shown to remove the surface defects of fiber and attenuate the fiber diameter to promote more uniform heat treatment during pyrolysis.
The above studies have focussed on increasing the strength of the membranes by optimisation of pyrolysis conditions or of the precursors.
There is a need to provide a method for forming membranes with enhanced selectivity in gas separation.